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Published on August 28, 2000

Tire pressure's role key in recall

While no one knows the technical problems, if any, that led Bridgestone/Firestone Inc. to recall 6.5 million light truck tires, some tire dealers have suspected one possible factor from the start.

Even before the Aug. 9 recall was announced as stories about alleged problems with the tires came to light, a number of dealers were telling us to look into the inflation pressures of the tires in question.

It's well known that many, if not most, tires are run with less air pressure than what the vehicle maker recommends. The reason is that most drivers rarely check their tires, all of which lose pressure over time.

Dealers said many of the tires they see on their customers' vehicles are four to six psi under the recommended pressure, found on every vehicle's door post and in the owner's manual. And inflation pressure matters.

Amid the recall hysteria, this is something that hasn't been made clear to motorists, and not just those with recalled Firestone tires. Under-inflated tires generate increased heat due to friction which, if the tire gets hot enough, can lead to rubber degradation and failure. This is important to understand in the case of a recall in which tire treads separated.

Prior to the recall announcement, the owners of Ford Explorers—ve-hicles that figure prominently in the National Highway Traffic Safety Administration's probe of the tires—were advised by the auto maker to inflate their tires to 26 psi.

At 26 psi, a set of four P235/75R15 standard load tires have a maximum load carrying capacity of slightly more than 7,000 pounds, according to the Tire and Rim Association (TRA) Yearbook.

The gross vehicle weight of a typical, model year 2000 four-door Explorer is about 5,300 pounds. That leaves about 1,700 pounds to accommodate passengers, baggage and other types of weight, such as trailer pulling.

While this may be sufficient if the inflation pressure is maintained, it invites trouble if the motorist follows the typical pattern of ignoring his or her tires and allowing air pressure to drop.

If inflation pressure falls to 20 psi, for example, the all important difference between the Ford Explorer's gross weight and load-carrying capacity of the tires drops to about 870 pounds, the TRA tables show.

At this level, it doesn't take much to overload the tires, and overloaded tires will generate serious heat within minutes.

Add to this the potential of high-speed driving, another heat generator, and scorching road surfaces, which can reach 170 degrees Fahrenheit in some areas, and you have a recipe for potential disaster. This scenario applies to all vehicles and tires, not just Ford Explorers and Firestones.

As investigators look into the causes of the tread separations found on the recalled tires, the roles played by inflation pressure and overloading should get close scrutiny.


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